Casual Marijuana Use Linked to Brain Changes in Young Adults

Marijuana is the most commonly used illicit drug in the United States. Recent studies have shown that heavy marijuana use by young adults is associated with changes in brain structure linked to a loss of motivation, increased anxiety, and cognitive impairments.

A study from April 2014 has found that even light use of marijuana—smoking pot once a week—may cause structural changes in the size and shape of two brain regions. This is the first study to show casual use of marijuana may be related to major brain changes.

The study titled “Cannabis Use is Quantitatively Associated with Nucleus Accumbens and Amygdala Abnormalities in Young Adult Recreational Users” was published April 16 in The Journal of Neuroscience. The researchers found a direct correlation between the quantity of cannabis use and the degree of changes to the developing brain of people under 25-years-old.

Marijuana Use Can Alter the Amygdala and Nucleus Accumbens

The more pot a young adult smokes the more the brain is altered. The findings suggest that marijuana exposure, even in young recreational users, is associated with exposure-dependent alterations of the neural matrix of core reward structures and is consistent with animal studies of changes in the gray matter volume of the amygdala and nucleus accumbens.

In the recent study, Jodi Gilman, PhD, Anne Blood, PhD, and Hans Breiter, MD, of Northwestern University and Massachusetts General Hospital/Harvard Medical School used magnetic resonance imaging (MRI) to compare the brains of 18- to 25-year olds who reported smoking marijuana at least once per week with people who had a minimal to zero history of marijuana use.

The team of scientists compared the size, shape, and density of the nucleus accumbens and the amygdala—a brain region that plays a central role in anxiety and emotional regulation—in 20 marijuana users and 20 non-users. Each marijuana user was asked to estimate their drug consumption over a three-month period, including the number of days they smoked and the amount of the drug consumed each day.

The MRI brain images showed significant brain differences even in light marijuana users. In particular, the nucleus accumbens—which is a brain region linked to reward processing and motivation—was larger and altered in its shape and structure in the marijuana users compared to non-users. Of particular interest to the researchers was that the nucleus accumbens was abnormally large, and its alteration in size, shape and density was directly correlated to how many joints an individual had smoked.

The researchers were unable to assess the THC content of the marijuana each person smoked because the study was retrospective. In general, THC levels are estimated to range from 5 to 9 percent or even higher in currently available marijuana. As a note of interest, the THC of marijuana during the 1960s and 1970s was generally around 1 to 3 percent.

Medical Marijuana Has Many Pros

In recent months, I have written a few Psychology Today blog posts based on studies linking heavy marijuana use to negative consequences in young adults. I have received a wide range of heartfelt personal emails from parents of children and individuals who have benefitted from the use of medical marijuana and attest that the pros to medicinal cannabis greatly outweigh the cons.

Clearly, cannabis has many beneficial medicinal purposes. Also, many adults I know use marijuana casually and don't appear to suffer any dramatic negative consequences. The human brain is still developing until age 25 which makes it particularly vulnerable to exogenous substances. Car rental companies base their decision not to rent vehicles to people under 25 based on neuroscience and the vulnerabilities of a human brain that is still developing.

Although I haven’t come across new research on the pros of medical marijuana, I wanted to balance my writing on the pros and cons of marijuana by including a link to 105 peer-reviewed "Medical Studies Involving the medical use of Cannabis and Cannabis Extracts between 1990 - 2012."

Conclusion: More Research on Light Marijuana Use Is Needed

The researchers conclude that this study challenges the idea that casual marijuana use doesn’t have negative consequences on brain development for young adults. Many young people assume that getting high occasionally won’t have any harmful effects. The researchers hope this study will make young adults think twice before smoking marijuana even once a week.

The researchers believe their data suggests even casual marijuana use causes brain changes but acknowledge that further research, including longitudinal studies, is needed.

If you'd like to read more on this topic, check out my Psychology Today blog posts:

>> Of particular interest to the researchers was that the nucleus accumbens was abnormally large, and its alteration in size, shape and density was directly correlated to how many joints an individual had smoked.

You are to be commended for sontuing to educate young people as to the dangers inherent in consumption of cannabis. I myself was addicted to that substance for many years, and I can confirm how terrible a problem it is.

While under the influence of cannabis, I did and thought all kinds of stupid, pointless, destructive and even dangerius things.

It starts small, with listening to Pink Floyd while watching "The Wizard of Oz," but it always escalates. Soon, you start thinking that Bruce Springsteen's "Blinded By the Light" makes sense. Then you start thinking that Lou Reed could sing. Allen Ginsberg starts to seem like a genius. You get so hungry that you'll actually eat Circus Peanuts and say, "THese aren't bad at all." Then you find yourself thinking that "Big Bang Theory" is funny. Eventually, you may even convince yourself that having a dude's whangus in your rectum isn't such a gross idea at all.

I am so fascinated by the cultural impact weed has generated. NBC did a poll on millenials and their perceptions about what is more dangerous or not. Unsurprisingly, weed was ranked less dangerous than tobacco, alcohol and sugar. Marijuana was considered more safer than sugar or alcohol. How this change in perception evolve throughout the decade is something I think needs to get looked at. Because the more you believe it is safe, the longer you will hold that conviction. And with more and more celebrities -- especially young celebrities-- endorsing the casual use of the drug, the drugs popularity will only get enhanced. Hollywood, too, is partly responsible.

Correlation does not equal causation. Correlation does not equal causation. Correlation does not equal causation.

How many times does this need to be stated?

When two variables, such as the size of the nucleus accumbens and marijuana use, are found to be related, one possiblity is that one variable caused the other. For example, marijuana use could affect the size of the NA. Alternatively, a larger-than-average nucleus acumbens could cause people to smoke marijuana.

The other possibility is that a third variable caused both. A particular genetic profile, for example, could produce both a large NA and an enjoyment of marijuana. Or both marijuana use and a large NA could be caused by environmental factors, or any one of an infinite number of other third variables. The problem with correlational research, unlike experiments, is that it is impossible to rule out ALL the third variables.

Christopher, this is not the first time that you've reported on correlational research and stated that it provides evidence of causation. It makes you sound silly.

This study has previously been proven as an ineffective, poorly designed study. The data was manipulated to distract from lack of significance. To small a sample size. The author should be held accountable for this degree of misinformation. It's just not that bad for you. Just try to keep it away from kids like we do most things... I still think sugar and acetaminophen are still much more dangerous..

I posted a portion of this comment on another article that referenced this particular study..

Let's take a closer look at this and why this study and any other published work that includes references to it should be viewed with a great deal of scepticism. The marijuana study included a total of 40 people, yes I said 40 not 400 or 4000; with only 20 of those included being actual pot smokers age 18-25.

Dig a little deeper and you will find that the tolerances used to measure the amount of weed the 20 smokers in the study consumed daily was objective at best. Tolerances did not take into account any other drugs, alcohol or substances that the control group of 20 smokers used in addition to the marijuana. As shaky as these findings may be the National Institute on Drug Abuse has jumped on this as if it were earth shaking news to be viewed as a credible assessment of marijuana's impact on the developing human brain of people under the age of 26.

Only one media outlet embraced the study and gave it any attention at all, Fox News. They devoted 24 hours of editorial comment from each of their news anchors to supporting the study. Obviously they had to protect an agenda that would prevent marijuana from being legalized. They rake in a lot of revenue from the pharma, liquor and tobacco lobby, all would be hit hard if marijuana was legalized nationally.

Thank you everyone for joining the discussion and sharing your thoughts on this hot-button topic.

I actually just Googled "Casual Use of Marijuana" to find that this research study has been covered by a wide range of media outlets since the report was published in The Journal of Neuroscience on April 16.

In writing this blog post I took into consideration different viewpoints and was very clear in my conclusion that more research on the casual use of cannabis is needed.

Also, to keep this blog post fair and balanced I included a direct link to 105 peer-reviewed studies on marijuana under the heading "Medical Marijuana Has Many Pros." In case anyone missed the link the first time they read my blog post here it is again: